You may be able to download the PSE8 trial and then validate it with your serial number, but you will need patience as the servers are quite slow. You must first click on the link at the bottom for the current PSE11 trial page, then sign in (top right with your Adobe ID and password)

Then copy this PSE8 link into the same browser window and save the DMG file. Good luck.

The thing is that for PSE 8 downloads and boxed versions have incompatible serial numbers, so the trial won't help you any. In those days when you bought PSE in a box, you had to uninstall the trial and reinstall it from disc.